Novel catalyst systems provide cationically photocured compositions with reduced color, improved degree of cure, and increased thermal stability. Such compositions are useful as optically colorless or low color coatings, adhesives, encapsulants, sealants and abrasives for optical and electronic applications.
Salts that have an organic, inorganic or organometallic cation and a nonnucleophilic counteranion are widely used in industry as initiators, particularly photoinitiators, for cationic addition polymerization reactions. Common initiator salts include onium salts such as diaryliodonium, triarylsulfonium, and (cyclopentadienyl)(arene)iron+ salts of the anions PF6xe2x88x92, AsF6xe2x88x92, or SbF6xe2x88x92. It has been well documented that the identity of the anion associated with the onium cation can significantly affect the rate of polymerization as well as the color and thermal stability of the resulting cured polymer. For example when a sulfonium SbF6xe2x88x92 salt is used to catalyze photocationic addition polymerization reactions, highly colored cured materials are generally produced whereas when a sulfonium PF6xe2x88x92 salt is used as the catalyst essentially colorless polymers result. Measurement of the degree of cure of these two systems, however reveals that while the sulfonium PF6xe2x88x92 catalyst produces a colorless product there is a high level of unreacted epoxy monomer present in it, producing thermal instability in the cured product. And the sulfonium SbF6xe2x88x92 salt, while producing a highly colored material, has essentially no unreacted epoxy as determined by infrared spectroscopy.
Recently it has been found that polymers catalyzed by anions having three highly fluorinated alkylsulfonyl, fluorinated arylsulfonyl or perfluoroalkylsulfonyl groups, and combinations thereof, exhibit very strong catalytic activity. For example, epoxy formulations that have a tris-trifluorosulfonylmethide (xe2x80x9cmethidexe2x80x9d) anion provide very rapid cures in cationic polymerization reactions. Although methide-containing initiators seem to be a direct replacement for SbF6xe2x88x92 anion-containing initiators in cationic addition polymerization reaction systems as far as cure rate is concerned, these systems impart significantly reduced thermal stability to the cured material.
Briefly, the present invention provides a curable composition comprising
a. at least one cationically curable monomer, and
b. a curing agent comprising Photocurative A or Photocurative B, wherein
Photocurative A comprises
1) an effective amount of an accelerating photochemically active salt comprising a photochemically active cation and an accelerating anion, and
2) an effective amount of an inhibiting non-photochemically active salt comprising a non-photochemically active cation and an inhibiting anion; and
Photocurative B comprises
1) an effective amount of an inhibiting photochemically active salt comprising a photochemically active cation and an inhibiting anion, and
2) an effective amount of an accelerating non-photochemically active salt comprising a non-photochemically active cation and an accelerating anion;
wherein the inhibiting and accelerating anions are defined by photo differential scanning calorimetry (pDSC) and thermal differential scanning calorimetry (tDSC) such that anions of triarylsulfonium salts that have a total DSC exotherm energy value in the range of 0 to 300 Joules per gram (J/g), preferably 0 to 250 J/g, and more preferably 0 to 200 J/g, are classified as inhibiting anions, and anions of triarylsulfonium salts that have a total DSC exotherm energy value in the range of greater than 300 to 750 Joules per gram (J/g), preferably 325 to 600 J/g, and more preferably 350 to 500 J/g, are classified as accelerating anions.
In another aspect, the present invention provides curatives for producing high performance, cationically cured compositions, the curatives being designated Photocurative A and Photocurative B, as defined above.
In yet another aspect, the present invention provides a cured composition comprising the curable composition described above that has been subjected to curing conditions. These compositions preferably have one or more of the following characteristics: increased degree of cure, increased Tg, increased thermal stability, and reduced color or colorless cured compositions compared to conventional cationically cured compositions.
In a further aspect, the present invention provides a method for photopolymerizing a cationically polymerizable composition comprising Photocurative A or Photocurative B, as defined above, and subjecting the composition to photocuring conditions to produce high performance compositions.
Cationically polymerizable materials that can be cured or polymerized by the curatives of the present invention, using the latter in a catalytically effective amount, are those known to undergo cationic polymerization and include 1,2-, 1,3-, and 1,4-cyclic ethers (also designated as 1,2-, 1,3-, and 1,4-epoxides), vinyl ethers, N-vinyl compounds, ethylenically unsaturated hydrocarbons, cyclic formals, and cyclic organosiloxanes. An extensive list of cationically polymerizable monomers which can be used in this invention are given in U.S. Pat. Nos. 3,347,676 and 3,842,019, which patents are incorporated herein by reference.
We have found that the addition of additional anions to a conventional photochemically active initiator salt can improve the properties of a cationically cured composition depending upon the nature of the added anion and the anion of the photochemically active salt. It is the added anion that produces the effect. The anion can have any cation associated with it that does not interfere with the photochemical activity of the initiator salt.
This invention provides an improved catalyst system for cationically photocurable compositions. The invention utilizes conventional cationic initiators with the addition of effective amounts of additional anions. Choice of optimal initiator/anion combinations provide compositions with high performance characterics not previously achievable using conventional curatives. Colorless or low color compositions of the present invention can represent an improvement in the art compared to compositions made with conventional cationic initiator systems that can impart a deep yellow/brown color to the cured composition or have poor curing characteristics.
In this application:
xe2x80x9ccolorlessxe2x80x9d, xe2x80x9clow colorxe2x80x9d, and xe2x80x9creduced colorxe2x80x9d means colorless to the naked eye, or reduced or lower in color compared to a corresponding polymer cured by a conventional photocurative. Conventional photocuratives are those known in the art to cure cationic monomers. Examples of conventional curatives include the photoinitiators mentioned in the background of this application and are disclosed, for example, in U.S. Pat. Nos. 4,250,311; 3,708,296; 4,069,055; 4,216,288; 5,084,586; 5,124,417; 4,985,340; 5,089,536; and 6,025,406;
xe2x80x9chigh performancexe2x80x9d means having at least one of the following characteristics: increased degree of cure, increased Tg, increased thermal stability, reduced color or colorless cured compositions compared to conventional compositions;
xe2x80x9cinitiatorxe2x80x9d means a photochemically active salt;
xe2x80x9cnormal cure conditionsxe2x80x9d is defmed as comprising the steps of exposing the sample to light, generally on the order of 1 to 2 Joules of UVA and if desired heating the composition to between 100 to 150xc2x0 C. to complete cure;
xe2x80x9cphotochemically active compositionsxe2x80x9d are those that are cationically curable, such as epoxy and epoxy containing materials, and comprise a photochemically active salt;
xe2x80x9cphotochemically active saltxe2x80x9d is one that, upon photolysis, produces an acid (can be a Bronsted (H+) or Lewis acid (M+n)) capable of initiating polymerization of cationically curable material;
xe2x80x9cresin mixturexe2x80x9d means a mixture of curable materials without a photochemically active salt;
xe2x80x9csubstituentxe2x80x9d or xe2x80x9csubstitutedxe2x80x9d group means halogen or any other group that doesn""t interfere with the polymerization or intended properties of the cured polymer;
xe2x80x9ctotal DSC exotherm energyxe2x80x9d means the sum of the photo DSC exotherm energy and the thermal DSC exotherm energy; and
xe2x80x9cUVAxe2x80x9d means electromagnetic radiation between 320 nm to 390 nm in wavelength.
The present invention provides advantages in cationically cured compositions not heretofore seen in the art. Higher performance compositions (higher Tg values, higher degree of cure, better thermal stability, etc.) that can be colorless or low color can be obtained that represent in improvement in the art compared to conventional single initiator systems that impart a deep yellow/brown color to the cured composition or have poor curing characteristics.
The invention provides cured polymers having one or more of the following characteristics: Tg values (determined by DMA at 1 Hz as explained below) preferably at least 10xc2x0 C. higher, more preferably at least 25xc2x0 C. higher, and most preferably at least 50xc2x0 C. higher than the Tg of corresponding polymers cured by conventional curatives; percent weight loss (by TGA) preferably less than 10%, more preferably less than 8% and most preferably less than 6%; and degree of cure (measured by IR) of preferably greater than 80%, more preferably greater than 90%, and most preferably greater than 95%.
Catalysts for cationic addition photopolymerization reactions that produce compositions with long dark stability (before light exposure) and result in highly cured, colorless or low color, thermally stable products can be very useful in optically colorless coatings, adhesives, encapsulants, and sealants for optical and electronic applications. Improved dark stability can allow greater latitude in thermal processing of curable/thermoplastic blends, allowing the use of higher temperature thermoplastics or higher processing temperatures for lower viscosity extrusion conditions. Improved thermal stability of the cured compositions can allow higher operating temperatures of these materials and lower levels of outgassing that can be important to critical applications such as electronics (hard disk drives, for example) or in places that require stability in extreme environments (such as automotive applications). Improved thermal stability of the cured compositions can also be important to products used under stressful conditions, such as abrasives, where the use of the product generates heat that can contribute to product failure.